A fuel cell is a kind of power generation device which converts chemical energy. In fuel into electrical energy by an electrochemical reaction within a fuel cell stack without converting the chemical energy to heat by combustion. This type of power generation device may be used not only to supply power for an industry, a household, and vehicle driving but also to supply power to small electric/electronic products, in particular, portable devices.
Currently, as a power supply source for driving a vehicle, polymer electrolyte membrane fuel cells (PEMFC) and proton exchange membrane fuel cells (PEMFC) having the highest power density among the fuel cells have been most researched, because these cell types have a rapid starting time and a rapid power conversion reaction time due to a low operating temperature.
The polymer electrolyte membrane fuel cell is configured to include a membrane electrode assembly (MEA) in which catalytic electrode layers where the electrochemical reaction is generated are attached to both sides of a solid polymer electrolyte membrane in which protons move, a gas diffusion layer (GDL) which serves to uniformly distribute reaction gases and transfer generated electrical energy, a gasket and a fastener which maintain air tightness and proper fastening pressure of the reaction gases and cooling water, and a bipolar plate which moves the reaction gases and the cooling water.
In assembling the fuel cell stack using such unit cell components, a combination of the membrane electrode assembly and the gas diffusion layer which are main components is positioned at the innermost portion of the cell. Here, the membrane electrode assembly is provided with the catalytic electrode layers, i.e., an anode and a cathode which are applied with a catalyst to provide a reaction of hydrogen and oxygen at both surfaces of the polymer electrolyte membrane, and an outer portion where the anode and the cathode are positioned is stacked with the gas diffusion layer, a gasket, and the like.
An outer portion of the gas diffusion layer is supplied with reaction gas (hydrogen which is fuel and oxygen or air which is an oxidizer) and is provided with a bipolar plate formed with a flow field through which cooling water passes.
A plurality of unit cells are stacked by setting the configuration as a unit cell, and then the outermost portion thereof is coupled with a current collector, an insulating plate, and an end plate for supporting the stacked cells. The unit cells between the end plates are repeatedly stacked and fastened, thereby configuring the fuel cell stack.
To obtain necessary potential in a vehicle, the unit cells need to be stacked as much as necessary. Since potential generated from one unit cell is about 1.3 V, a plurality of cells are stacked in series to produce power required to drive a vehicle.
In a fuel cell vehicle, the impedance of the fuel cell stack is used to measure an AC voltage of the stack.
The typical apparatus for measuring impedance of a fuel cell stack applies an alternating current (AC) (e.g., sinewave) to a fuel cell stack and then measures an AC voltage of the fuel cell stack and uses the measured AC voltage and AC to measure the impedance of the fuel cell stack.
The typical apparatus for measuring impedance of the fuel cell stack measures the impedance of a fuel cell stack while ignoring the fact that the impedance of the fuel cell stack changes when the DC current of the fuel cell stack chances depending on fluctuation of a load.